Adjust lifetimes on ModelIter to make them more permissive

[sgpthomas]

Currently, iterating over a Model creates FuncDecls that reference the lifetime of the reference of the model that they come from, rather than the lifetime of context it contains.

This means that it is impossible to use those FuncDecls after the model is dropped, even when the context is still around. From looking at the code, this just seems like a bug, but let me know if this is intentional.

This PR adjusts the lifetimes of ModelIter to also explicitly include the lifetime of the model reference. This way it can return FuncDecl that properly reference the lifetime of the context.

[waywardmonkeys]

I think this looks good.

It would be good to have a test for this to at least compile in a way that didn't before so that we make sure that this doesn't regress.

[sgpthomas]

Added a test that only compiles with this change. Let me know if it looks good to you. Also, is the wasm build something that I broke? Or can I ignore that?

[waywardmonkeys]

Looks great and will merge it shortly.

I don't think you broke the WebAssembly build. I'll try to take a look at it today or tomorrow.

Thanks for the PR!